1. Field of the Invention
The present invention relates to a solid-state image sensor and an imaging apparatus equipped with the solid-state image sensor.
2. Description of Related Art
Today, imaging apparatuses such as video cameras and electronic still cameras are widely used by consumers. Such a camera typically includes a solid-state image sensor which may be, for instance, a CCD sensor or an amplifier-type sensor. The solid-state image sensor is constituted with a plurality of pixels, each equipped with a photoelectric conversion unit that generates a signal charge in correspondence to the amount of incoming light, arrayed in a matrix pattern.
In an amplifier-type solid-state image sensor, the signal charge having been generated and stored at the photoelectric conversion unit at each pixel is guided to an amplifier unit at the pixel and the signal amplified at the amplifier unit is then output from the pixel to a vertical signal line. Japanese Laid Open Patent Publication No. H11-177076 and Japanese Laid Open Patent Publication No. 2004-335882 disclose that the amplifier unit in the amplifier-type solid-state image sensor may be constituted with a junction field effect transistor, or Japanese Laid Open Patent Publication No. 2004-111590 discloses that the amplifier type solid-state image sensor may be a CMOS solid-state image sensor equipped with MOS transistors constituting the amplifier units thereof. In the solid-state image sensor equipped with amplifier units each constituted with a JFET, the gate area of the JFET is used as a charge/voltage conversion unit that takes in the charge generated in the photoelectric conversion unit and converts the charge to a voltage. A floating diffusion functions as the charge/voltage conversion unit in a solid-state image sensor equipped with amplifier units constituted with MOS transistors.
In an imaging apparatus such as a camera, the focusing condition at the photographic lens must be detected in order to enable autofocus adjustment. The focusing condition is detected by a special focus detection element disposed in addition to the solid-state image sensor in the related art. However, an imaging apparatus adopting this structure, which includes the focus detection element and a focus detection optical system used to guide light to the focus detection element, will be a bulky and costly apparatus.
Accordingly, Japanese Laid Open Patent Publication No. 2003-244712 and Japanese Laid Open Patent Publication No. 2000-292686 disclose that solid-state image sensors assuming a structure that allows it to also function as a focus detection element adopting a split-pupil phase-difference focus detection method (may otherwise be referred to as a split-pupil method or a phase-difference method). Through the split-pupil phase-difference method, the extent of defocusing at the photographic lens is detected by splitting a light flux passing through the photographic lens, thereby forming a pair of split images and detecting the amount to which the patterns in the split images are offsets relative to each other (the amount of phase shift).
The solid-state image sensors disclosed in Japanese Laid Open Patent Publication No. 2003-244712 and Japanese Laid Open Patent Publication No. 2000-292686 each include a plurality of focus detection pixels via which focus detection signals to be used to detect the focusing condition are generated, disposed therein in addition to imaging pixels that output imaging signals used to form image signals expressing a subject image. Each imaging pixel, equipped with a single photoelectric conversion unit, includes a color filter disposed thereat and outputs a signal resulting from photoelectric conversion executed on a light flux received from an area of the exit pupil of the photographic lens, not substantially offset from the center of the exit pupil. The focus detection pixels at the solid-state image sensor disclosed in patent reference 4 or 5, each equipped with two separate photoelectric conversion units, do not include color filters disposed thereat. No color filters are disposed at the focus detection pixels in order to increase the SN ratio in the focus detection signals by maximizing the quantity of light entering therein without imposing any restrictions with regard to the incoming wavelength and thus improve the focus detection accuracy. Over the photoelectric conversion units, micro lenses are disposed each in correspondence to one of the pixels. The two separate photoelectric conversion units at each focus detection pixel are disposed at a position at which images of the photoelectric conversion units are substantially formed on the exit pupil of the photographic lens via the microlens at a position substantially conjugate with the position of the exit pupil of the photographic lens. This means that since the distance between the exit pupil of the photographic lens and the micro lens is sufficiently long in relation to the size of the micro lens, the two separate photoelectric conversion units are disposed at a position substantially matching the focusing plane of the micro lens. At each focus detection pixel assuming the positional relationship described above, one of the two photoelectric conversion units selectively receives a light flux transmitted through an area ranging over part of the exit pupil of the photographic lens and offset along a specific direction from the center of the exit pupil, and thus performs photoelectric conversion. The other photoelectric conversion unit at the focus detection pixel selectively receives a light flux transmitted through an area ranging over part of the exit pupil of the photographic lens and offset along the opposite direction from the center of the exit pupil and performs photoelectric conversion.
The signals output from the focus detection pixels in the solid-state image sensor disclosed in Japanese Laid Open Patent Publication No. 2003-244712 and Japanese Laid Open Patent Publication No. 2000-292686, with no color filters disposed thereat, cannot be used as imaging signals based upon which image signals expressing the subject image are formed. Thus, a condition akin to that caused by the absence of a pixel is induced at the individual focus detection pixels during imaging operation. Accordingly, simulated image signals are generated through processing referred to as interpolation executed for areas where the focus detection pixels are disposed. Japanese Laid Open Patent Publication No. 2001-251636 discloses that the interpolation processing is normally executed in order to generate a simulated image signal in correspondence to a defective pixel by determining the defective pixel from which no signal is not output or where a significant level of noise is present and by using signals output from normal pixels in the surrounding area. For instance, if a normal signal is output from a given signal that should generate a blue (B) signal, the signals from four normal B pixels present in the surrounding area are averaged and the averaged signal value is used as the signal value of the simulated image signal corresponding to the defective pixel.
It is to be noted that focus detection pixels are often disposed over specific local areas, e.g., over a top area and a bottom area, a left-side area, a right-side area or a central area, as disclosed in Japanese Laid Open Patent Publication No. 2000-292686, rather than evenly over the entire effective pixel area at the solid-state at the solid-state image sensor as disclosed in Japanese Laid Open Patent Publication No. 2003-244712. This is because if the focus detection pixels are evenly distributed over the entire effective pixel area, the number of imaging pixels is bound to decrease to result in a lower image quality and the load of the interpolation processing described earlier increases.